The present invention relates to improvements in the controlled heat treating of products and in particular the controlled heat treating of products by means of a fluidized bed heated from the exterior thereof.
The improved heating of a fluidized bed furnace is suitable for use in the continuous processing of a strip configured product by passing the product through the furnace and is also suitable for the batch treating of products at high temperatures, for example high speed cutting steels. Infrared radiation lamps directly heat a boundary layer of particles of the fluidized bed and the excellent thermal transfer characteristics of the bed cause the rapid dissipation of this energy throughout the bed. Thus, the radiation emitted by the lamps is radiated to the particles of the bed forming a boundary layer whereafter heat is dissipated throughout the bed by the action of the bed.
A number of advantages result from this unique combination such as the heat treating of high speed steel products, in a non contaminating fluidized bed at temperatures in the range of 1100.degree. C. to 1300.degree. C.; excellent control with respect to the heat energy being imparted to the bed by varying the electrical energy to the lamps and thereby directly vary the radiation absorbed by the exposed boundary layer of the particles; and improved start up characteristics whereby delays experienced in bringing a fluidized bed to operating temperature are reduced.
Heat treatment of metals in fluidized beds with or without a controlled atmosphere have been proposed for many applications and the history of fluidized beds and the various techniques are well described in an article by R. W. Reynoldson, published in the publication Heat Treatment of Metals, 1977.1, entitled Control Atmosphere Fluidized Bed for Heat Treatment of Metals. In this article, a fluidized bed furnace is shown where internal resistance heating elements are placed within a fluidized bed for the batch treating of a product. The article also refers to various types of combustion processes which may be carried out within and/or above a fluidized bed. Generally, fluidized beds heated by combustion have an operating temperature somewhere between 600.degree. and 800.degree. C. and are not suitable for high temperature application.
In fluidized beds and in the heat treating of any product, it is important to provide uniform heating and it is also important to provide a system which is capable of fast start up.
A substantial amount of research has been undertaken with respect to introducing of a combustible gas to a fluidized bed in a manner where the gas assists in the fluidization of the bed while also providing the proper gas mixture to support combustion within the bed. In cases where a controlled atmosphere is required, the combustion process is still carried out within a portion to heat some of the particles which are then stripped from the combustion gases and returned to the fluidized bed for heating of the remaining particles of the bed and heating of the product being treated. In this way, the product being treated is isolated from the atmosphere required to support combustion.
The demands placed on the heating source of the fluidized bed should be capable of rapidly raising the temperature of the fluid bed from ambient to the heat treat temperature, preferrably up to about 1200.degree. C. In addition, the heating source should be capable of providing uniform heating of the bed once the bed has arrived at its operating temperature. To date, the ability to rapidly raise the temperature of the fluid bed to operating levels and thereafter maintain it in a manner such that the uniform heating of products is accomplished, has been achieved with fluidized beds, but problems occur with gas fired beds at the upper temperatures.
Heat treating at high temperatures in the order of 1100.degree. C. to 1300.degree. C. and particularly heat treating of high speed steel, has been generally confined to molten salt baths which are hazardous materials to the environment and are dangerous in the work place. Some attempts have been made to use fluidized beds for high temperature heat treating of high speed steel. One proposal with respect to fluidized beds is to use electrically conductive fluid bed particles and to heat the fluidized bed electrically using the electrical resistance of the particles. This method is not suitable with materials to be treated which are conductive.
High temperature fluidized bed are possible using externally electrically heated retorts. Electrical resistance heaters are disposed about the exterior of the retort to heat the walls thereof which in turn heat the fluid bed particulate. Such beds have long startup times and the life expectancy of the electrical heaters is short at these high temperatures. If metal retorts are used, problems with breakdown can occur.